Noise and Electrical Distortion

Because of the very high switching rate and relatively low signal strength found on data, address, and other buses within a computer, direct extension of the buses beyond the confines of the main circuit board or plug-in boards would pose serious problems. First, long runs of electrical conductors, either on printed circuit boards or through cables, act like receiving antennas for electrical noise radiated by motors, switches, and electronic circuits:

Such noise becomes progressively worse as the length increases, and may eventually impose an unacceptable error rate on the bus signals. Just a single bit error in transferring an instruction code from memory to a microprocessor chip may cause an invalid instruction to be introduced into the instruction stream, in turn causing the computer to totally cease operation.
A second problem involves the distortion of electrical signals as they pass through metallic conductors. Signals that start at the source as clean, rectangular pulses may be received as rounded pulses with ringing at the rising and falling edges:

These effects are properties of transmission through metallic conductors, and become more pronounced as the conductor length increases. To compensate for distortion, signal power must be increased or the transmission rate decreased.
Special amplifier circuits are designed for transmitting direct (unmodulated) digital signals through cables. For the relatively short distances between components on a printed circuit board or along a computer backplane, the amplifiers are in simple IC chips that operate from standard +5v power. The normal output voltage from the amplifier for logic '1' is slightly higher than the minimum needed to pass the logic '1' threshold. Correspondingly for logic '0', it is slightly lower. The difference between the actual output voltage and the threshold value is referred to as the noise margin, and represents the amount of noise voltage that can be added to the signal without creating an error:

Asynchronous vs. Synchronous Transmission

Serialized data is not generally sent at a uniform rate through a channel. Instead, there is usually a burst of regularly spaced binary data bits followed by a pause, after which the data flow resumes. Packets of binary data are sent in this manner, possibly with variable-length pauses between packets, until the message has been fully transmitted. In order for the receiving end to know the proper moment to read individual binary bits from the channel, it must know exactly when a packet begins and how much time elapses between bits. When this timing information is known, the receiver is said to be synchronized with the transmitter, and accurate data transfer becomes possible. Failure to remain synchronized throughout a transmission will cause data to be corrupted or lost.
Two basic techniques are employed to ensure correct synchronization. In synchronous systems, separate channels are used to transmit data and timing information. The timing channel transmits clock pulses to the receiver. Upon receipt of a clock pulse, the receiver reads the data channel and latches the bit value found on the channel at that moment. The data channel is not read again until the next clock pulse arrives. Because the transmitter originates both the data and the timing pulses, the receiver will read the data channel only when told to do so by the transmitter (via the clock pulse), and synchronization is guaranteed.

Techniques exist to merge the timing signal with the data so that only a single channel is required. This is especially useful when synchronous transmissions are to be sent through a modem. Two methods in which a data signal is self-timed are nonreturn-to-zero and biphase Manchester coding. These both refer to methods for encoding a data stream into an electrical waveform for transmission.In asynchronous systems, a separate timing channel is not used. The transmitter and receiver must be preset in advance to an agreed-upon baud rate. A very accurate local oscillator within the receiver will then generate an internal clock signal that is equal to the transmitter's within a fraction of a percent. For the most common serial protocol, data is sent in small packets of 10 or 11 bits, eight of which constitute message information. When the channel is idle, the signal voltage corresponds to a continuous logic '1'. A data packet always begins with a logic '0' (the start bit) to signal the receiver that a transmission is starting. The start bit triggers an internal timer in the receiver that generates the needed clock pulses. Following the start bit, eight bits of message data are sent bit by bit at the agreed upon baud rate. The packet is concluded with a parity bit and stop bit. One complete packet is illustrated below:

Data Encryption.Privacy is a great concern in data communications. Faxed business letters can be intercepted at will through tapped phone lines or intercepted microwave transmissions without the knowledge of the sender or receiver. To increase the security of this and other data communications, including digitized telephone conversations, the binary codes representing data may be scrambled in such a way that unauthorized interception will produce an indecipherable sequence of characters. Authorized receive stations will be equipped with a decoder that enables the message to be restored. The process of scrambling, transmitting, and descrambling is known as encryption.
Custom integrated circuits have been designed to perform this task and are available at low cost. In some cases, they will be incorporated into the main circuitry of a data communications device and function without operator knowledge. In other cases, an external circuit is used so that the device, and its encrypting/decrypting technique, may be transported easily.

What is Data Communications?

The distance over which data moves within a computer may vary from a few thousandths of an inch, as is the case within a single IC chip, to as much as several feet along the backplane of the main circuit board. Over such small distances, digital data may be transmitted as direct, two-level electrical signals over simple copper conductors. Except for the fastest computers, circuit designers are not very concerned about the shape of the conductor or the analog characteristics of signal transmission.
Frequently, however, data must be sent beyond the local circuitry that constitutes a computer. In many cases, the distances involved may be enormous. Unfortunately, as the distance between the source of a message and its destination increases, accurate transmission becomes increasingly difficult. This results from the electrical distortion of signals traveling through long conductors, and from noise added to the signal as it propagates through a transmission medium. Although some precautions must be taken for data exchange within a computer, the biggest problems occur when data is transferred to devices outside the computer's circuitry. In this case, distortion and noise can become so severe that information is lost.
Data Communications concerns the transmission of digital messages to devices external to the message source. "External" devices are generally thought of as being independently powered circuitry that exists beyond the chassis of a computer or other digital message source. As a rule, the maximum permissible transmission rate of a message is directly proportional to signal power, and inversely proportional to channel noise. It is the aim of any communications system to provide the highest possible transmission rate at the lowest possible power and with the least possible noise

Communications Channels A communications channel is a pathway over which information can be conveyed. It may be defined by a physical wire that connects communicating devices, or by a radio, laser, or other radiated energy source that has no obvious physical presence. Information sent through a communications channel has a source from which the information originates, and a destination to which the information is delivered. Although information originates from a single source, there may be more than one destination, depending upon how many receive stations are linked to the channel and how much energy the transmitted signal possesses.
In a digital communications channel, the information is represented by individual data bits, which may be encapsulated into multibit message units. A byte, which consists of eight bits, is an example of a message unit that may be conveyed through a digital communications channel. A collection of bytes may itself be grouped into a frame or other higher-level message unit. Such multiple levels of encapsulation facilitate the handling of messages in a complex data communications network.

The message source is the transmitter, and the destination is the receiver. A channel whose direction of transmission is unchanging is referred to as a simplex channel. For example, a radio station is a simplex channel because it always transmits the signal to its listeners and never allows them to transmit back.
A half-duplex channel is a single physical channel in which the direction may be reversed. Messages may flow in two directions, but never at the same time, in a half-duplex system. In a telephone call, one party speaks while the other listens. After a pause, the other party speaks and the first party listens. Speaking simultaneously results in garbled sound that cannot be understood.
A full-duplex channel allows simultaneous message exchange in both directions. It really consists of two simplex channels, a forward channel and a reverse channel, linking the same points. The transmission rate of the reverse channel may be slower if it is used only for flow control of the forward channel.

Serial Communications.Most digital messages are vastly longer than just a few bits. Because it is neither practical nor economic to transfer all bits of a long message simultaneously, the message is broken into smaller parts and transmitted sequentially. Bit-serial transmission conveys a message one bit at a time through a channel. Each bit represents a part of the message. The individual bits are then reassembled at the destination to compose the message. In general, one channel will pass only one bit at a time. Thus, bit-serial transmission is necessary in data communications if only a single channel is available. Bit-serial transmission is normally just called serial transmission and is the chosen communications method in many computer peripherals.
Byte-serial transmission conveys eight bits at a time through eight parallel channels. Although the raw transfer rate is eight times faster than in bit-serial transmission, eight channels are needed, and the cost may be as much as eight times higher to transmit the message. When distances are short, it may nonetheless be both feasible and economic to use parallel channels in return for high data rates. The popular Centronics printer interface is a case where byte-serial transmission is used. As another example, it is common practice to use a 16-bit-wide data bus to transfer data between a microprocessor and memory chips; this provides the equivalent of 16 parallel channels. On the other hand, when communicating with a timesharing system over a modem, only a single channel is available, and bit-serial transmission is required. This figure illustrates these ideas:

Wireless Network

Wireless network pass on to any kind of computer network that is cable less and generally linked with a telecommunications and based on IEEE 802.11. Wireless Network can implement between nodes without any wire and remote information transmission system uses the radio waves, at the physical level of the network. There are several benefits of Wireless Network such as file sharing, Internet connection sharing, multi-player games, Internet telephone service, Computer mobility, and No unsightly wires. 802.11 support networking bandwidth and have an array of choices such as 802.11b, 802.11a,802.11g, 802.11n also known as Wi-Fi technologies and designed for precise networking programs. The hardware requirements of a Wireless network are Network adapter, Repeater, Network hub, and Modem.
There are different types of Wireless Networks such as LAN in which radio signals are used instead of wires and transmit data from one Pc to other Pc in the same network as it is Wi-Fi is a wireless network that enables connection to internet with WiFi functionality. it produced radio waves that can be picked up by WiFi receiver and fixed wireless data is also type of LAN that can be connect more then one building with the mean of sharing .The MAN have ability to connect several local area networks and a term WiMax used to represent wireless metropolitan area networks and Mobile devices networks contains Global System for Mobile Communications which is called GSM network and mainly used for cellular phones, Personal Communications Service is a radio band used to set up PCS and D-AMPS is an upgrade version of AMPS.
A Wireless network containing basic setup and to use a wireless network you have to need a networking card and Kernel config require IEEE 802.11 wireless network driver, Hardware Access Layer and ample Rate control algorithm. Wireless to crypto support modules .There are different kind of protocol used in a Wireless network, it has routing protocol such as DSDV, AODV, B.A.T.M.A.N, PWRP, DSR, OLSR, OORP, TORA,HSLS and The Ad-Hoc Configuration Protocol and Proactive Auto configuration. There are different related terms used for wireless network like WEP is a security protocol from IEEE, SSID, Static IP, DHCP, Subnet, LAN, WLAN, MAC, WAP, and Sniffer but the most commonly used term in these days is Wi-Fi such a technology wrapping the world.
Wireless network faces much security threat due to some reason as Spread spectrum used in LAN not very protected because the spreading codes openly therefore companies can intend the 802.11 mechanism and a hacker can easily demolish the security as it is a hacker can still snuffle the SSID. The use of DHCP in wireless network is also helpful for hacker to spoil your files because it automatically assigns IP addresses to users as they become active and hang the system. Generally networking security attack divided into two types which are passive and active .Passive including Eavesdropping, and Traffic analysis and active including Masquerading, Replay, Message modification, Denial-of-service. Loss if integrity and confident also cause of lack of security so it is necessary for all organizations that they make their system more secure even than before as no one can interrupt in it.

4G
4G - fourth generation, is also known as `Beyond 3G`! 4G will offer a total evolution in wireless comms and will allow users to get voice, data and multimedia whenever they want it, wherever they are - and at far higher streaming or transfer rates than ever experienced before. It’s expected to be working commercially by 2015, as the 3G networks are anticipated by then to be fully congested.
It’s hard to quantify yet exactly what 4G is, or will be. However it’s expected that it will be entirely IP based, and will be able to provide speeds of a whopping 100Mbit/s - 1 Gigabits, absolutely anywhere - of the highest quality, and with superior security attached. It will be able to offer products and services never seen before, and hopefully at reasonable prices (previous phone based services have often been expensive.) Certainly though, 4G should be able to offer streamed HD television - with movie downloads at around 5 minutes - as well as improved MMS (multimedia messaging service), mobile TV - as well as HD-TV content and video chat - all delivered `anytime, anywhere.
No doubt there will be stringent international standards attached for 4G, in the same way that there were European ones attached to 2G. Currently, various companies are claiming that they’re already in possession of this new 4G technology, but some commentators feel that this is misleading, and simply serves to confuse customer and investors. It does seem however that 4G is being most successfully championed in Japan, and in fact the first 4G phones may start to appear shortly. For now it’s a `wait and see` as companies battle it out over who can successfully launch the first genuine and robust 4G products to market - and consumers may increasingly will bide their time, particularly as each generation evolution means entirely replacing the previous generation’s mobile devices!
Who knows how long it will be until the first whispers of 5G will start to emerge? Answers on a postcard please! There are numerous online resources which will allow you to discover more about emerging trends

Wi-Fi
Wireless Fidelity – popularly known as Wi-Fi, developed on IEEE 802.11 standards, is the recent technology advancement in wireless communication. As the name indicates, WI-FI provides wireless access to applications and data across a radio network. WI-FI sets up numerous ways to build up a connection between the transmitter and the receiver such as DSSS, FHSS, IR – Infrared and OFDM. The development on WI-FI technology began in 1997 when the Institute of Electrical and Electronic Engineers (IEEE) introduced the 802.11 technology that carried higher capacities of data across the network. This greatly interested some of major brands across the globe such as the world famous Cisco Systems or 3COM. Initially, the price of Wi-Fi was very high but around in 2002, the IT market witnessed the arrival of a break through product that worked under the new 802.11 g standards. In 2003, IEEE sanctioned the standard and the world saw the creation of affordable Wi-Fi for the masses.
Wi-Fi provides its users with the liberty of connecting to the Internet from any place such as their home, office or a public place without the hassles of plugging in the wires. Wi-Fi is quicker than the conventional modem for accessing information over a large network. With the help of different amplifiers, the users can easily change their location without disruption in their network access. Wi-Fi devices are compliant with each other to grant efficient access of information to the user. Wi-Fi location where the users can connect to the wireless network is called a Wi-Fi hotspot. Through the Wi-Fi hotspot, the users can even enhance their home business as accessing information through Wi-Fi is simple. Accessing a wireless network through a hotspot in some cases is cost-free while in some it may carry additional charges. Many standard Wi-Fi devices such as PCI, miniPCI, USB, Cardbus and PC card, ExpressCard make the Wi-Fi experience convenient and pleasurable for the users. Distance from a wireless network can lessen the signal strength to quite an extent; some devices such as Ermanno Pietrosemoli and EsLaRed of Venezuela Distance are used for amplifying the signal strength of the network. These devices create an embedded system that corresponds with any other node on the Internet.
The market is flooded with various Wi-Fi software tools. Each of these tools is specifically designed for different types of networks, operating systems and usage type. For accessing multiple network platforms, Aircrack-ng is by far the best amongst its counterparts. The preferred Wi-Fi software tools list for Windows users is: KNSGEM II, NetStumbler, OmniPeek, Stumbverter, WiFi Hopper, APTools. Unix users should pick any of the following: Aircrack, Aircrack-ptw, AirSnort, CoWPAtty,Karma . Whereas, Mac users are presented with these options: MacStumble, KisMAC, Kismet. It is imperative for users to pick out a Wi-Fi software tool that is compatible with their computer and its dynamics.
Wi-Fi uses radio networks to transmit data between its users. Such networks are made up of cells that provide coverage across the network. The more the number of cells, the greater and stronger is the coverage on the radio network. The radio technology is a complete package deal as it offers a safe and consistent connectivity. Radio bands such as 2.4GHz and 5GHz depend on wireless hardware such Ethernet protocol and CSMA. Initially, Phase Shift Keying (PSK), a modulation method for conveying data was used, however now it has been replaced with CCK. Wi-Fi uses many spectrums such as FHSS and DSSS. The most popular Wi-Fi technology such as 802.11b operates on the range of 2.40 GHz up to 2.4835 GHz band. This provides a comprehensive platform for operating Bluetooth strategy, cellular phones, and other scientific equipments. While 802.11a technology has the range of 5.725 GHz to 5.850 GHz and provides up to 54 Mbps in speed. 802.11g technology is even better as it covers three non-overlapping channels and allows PBCC. 802.11e technology takes a fair lead by providing excellent streaming quality of video, audio, voice channels etc.
To connect to a Wi-Fi network an adapter card is essential. Additional knowledge about the SSID, infrastructure, and data encryption is also required. The Wi-Fi users don’t have to be concerned with the security issues. The security methods such as MAC ID filtering

CDMA

"Global EVDO Rev A subscriber numbers ramped up more than eightfold between Q2 07 and Q2 08," says ABI analyst Khor Hwai Lin. "The United States and South Korean markets show the highest growth rate for EVDO Rev A. The increased support for LTE from incumbent CDMA operators does not imply the imminent death of EVDO Rev A and B, because LTE is addressing different market needs compared to 3G."
EVDO Rev A subscribers will exceed 54 million by 2013 while Rev B subscribers will reach 25 million, reports ABI.
Over 31 million subscribers worldwide are already using HSDPA while 3.2 million subscribers were on HSUPA networks by Q2 08. Upgrades to HSUPA continue to take place aggressively around Western Europe and the Asia Pacific. Hence, HSUPA subscribers are estimated to hit 139 million by 2013.
"HSPA+ will contest with LTE and mobile WiMAX in the mobile broadband space," adds Asia-Pacific VP Jake Saunders. The 100Mbit/s download data rate difference between LTE (20MHz) and HSPA+ may not attract mid-tier operators to migrate, as LTE is based on OFDM technology that requires new components, while a move to HSPA+ is perceived to be more gradual transition."
Due to the large number of GSM 900 subscribers and the high possibility of refarming the spectrum for UMTS, ABI estimates that the majority of these global subscribers (about 1.2 billion by 2013) will be on 900MHz-only band. In second place would be dual-band users on 900MHz and 1,800MHz (1 billion by 2013). Subscribers of 2100MHz will ramp up steadily with a CAGR of 23.5 percent between 2007 and 2013.

Transition Networks Makes Remote CDMA Network Deployment Possible

A wireless telephone service provider was looking for an affordable solution to connect Nortel cellular switches over long distances. Nortel Networks’s cellular switches are only available with multimode fiber interfaces. These interfaces are used to connect their switches in a Central Office to a MicroCell switch in a Base Transmit Station, where the cellular antennas are located. This works well in densely populated areas where the Base Transmit Stations are located relatively close, within 2 km, to the Central Office.
The wireless service provider wanted to offer their services in rural areas. But the number of potential customers in these sectors didn’t justify the large capital expenditures required to install additional Central Offices and Base Transmit Stations. What they needed was a solution to connect the Base Transmit Stations back to their Central Office over distances greater then the multimode cable could handle.
Single mode fiber cable has the bandwidth capabilities to transmit signals over the distances required by the service provider. By utilizing Transition Networks’s® single mode to multimode 622Mbps converters, they were able to use single mode fiber cable to connect the Base Transmit Stations located up to 60km from the Central Office. The Transition Networks solution has allowed the service provider to save time and money in their network deployment, and reduced the hardware requirements to provide wireless services to customer in these remote cellular sectors.
In the diagram, Transition Networks’s 13-slot Point System Chassis, housing several single mode to multimode converters, was mounted in the same rack as the Nortel Cellular switch. Short multimode patch cables connected the switch to the media converters. Next the converters were connected to the single mode fiber installed between the central office and the various Base Transmit Stations located throughout the rural areas where the service provider wanted to offer their wireless services. Within each Base Transmit Station, a Transition Single Slot chassis and another media converter was installed to make the final connection to the Nortel MicroCell Switch which interfaces with the antennas.
The single mode to multimode converters offered by Transition Networks are not protocol specific, but are based on the data transmission speed. In this example, the Nortel equipment uses a proprietary protocol, which transmits at 634 Mbps and the Transition Networks converters were able to work with that data rate. Transition also offers similar converters designed to work in Fast Ethernet (100Mbps) and Gigabit Ethernet (1000Mbps) environments

Transition Networks is the leader in media conversion technology; offering a wide array of products including Ethernet, Fast Ethernet (FX and SX), Gigabit Ethernet, 10/100 rate converters, T1/E1, DS3, OC3, OC12, RS485, V.35, Token Ring and more. Our Point System chassis provides users with manageability, reliability, and future proofing. The Point System offers fully SNMP compliant read/write software including web-based management. The chassis also provides for redundant management, redundant power (AC or DC), converters that can be upgraded in the field, and more. Please contact Transition Networks for more information and how we may be able to help you deliver data services to your customers.

Wireless Sensor Networks

presents a comprehensive and tightly organized compilation of chapters that surveys many of the exciting research developments taking place in this field. Chapters are written by several of the leading researchers exclusively for this book. Authors address many of the key challenges faced in the design, analysis and deployment of wireless sensor networks. Included is coverage of low-cost sensor devices equipped with wireless interfaces, sensor network protocols for large scale sensor networks, data storage and compression techniques, security architectures and mechanisms, and many practical applications that relate to use in environmental, military, medical, industrial and home networks. The book is organized into six parts starting with basic concepts and energy efficient hardware design principles. The second part addresses networking protocols for sensor networks and describes medium access control, routing and transport protocols. In addition to networking, data management is an important challenge given the high volumes of data that are generated by sensor nodes. Part III is on data storage and manipulation in sensor networks, and part IV deals with security protocols and mechanisms for wireless sensor networks. Sensor network localization systems and network management techniques are covered in Part V. The final part focuses on target detection and habitat monitoring applications of sensor networks. This book is intended for researchers starting work in the field and for practitioners seeking a comprehensive overview of the various aspects of building a sensor network. It is also an invaluable reference resource for all wireless network professionals